In a fossil-fuelled power plant for generating electrical energy, owing to the combustion of a fossil fuel, a carbon dioxide-containing flue gas is formed. In addition to carbon dioxide, the flue gas contains further combustion products such as, for example, the gases sulfur dioxide and nitrogen dioxide, and solid particles, dust and soot. The flue gas is usually released into the atmosphere after a thorough separation of the solid components.
The increase in the fraction of carbon dioxide in the earth's atmosphere caused by humans, however, is considered to be responsible, as the main cause of the increase in the temperatures at the earth's surface called climate change. This is because carbon dioxide situated in the atmosphere prevents heat emission from the surface of the earth into space, which is generally termed the greenhouse effect.
In this respect, secondary measures suitable in existing power plants are being discussed in order to remove the carbon dioxide formed after the combustion from the flue gas. As a technical possibility for separating off carbon dioxide from the flue gas after a combustion process (post-combustion capture), for this purpose the flue gas is contacted with a scrubbing solution that is admixed with a suitable absorbent for carbon dioxide. In this case, the carbon dioxide is scrubbed out of the flue gas.
In a classical absorption-desorption process, the flue gas in this case is contacted in an absorption unit with the scrubbing solution, whereby, in particular, carbon dioxide is absorbed or reversibly bound. The flue gas purified in this manner is allowed out of the absorption unit, whereas the carbon dioxide-loaded scrubbing solution is passed into a desorption unit for separating off the carbon dioxide and regenerating the scrubbing solution. In the desorption unit, usually a thermal separation proceeds, that is to say the carbon dioxide is desorbed by supplying heat. The carbon dioxide is finally compressed, for example in many stages, and cooled and supplied to a store or utilization. The regenerated scrubbing solution is returned to the absorption unit, where it is available again for absorption of carbon dioxide.
In the context of such an absorption-desorption process, usually a chemical absorption using a basic scrubbing solution is used. In such an absorption, the acid flue gas components react with the basic absorbent present in the scrubbing solution. Currently, the most promising absorbents appear to be amine-containing absorbents, where, as amines, in particular alkanolamines, but also more complex sterically hindered amines having large alkyl groups, cyclic amines, amino acids or amino acid salts are used. The amines used form, together with carbon dioxide, either carbamates, or the carbon dioxide reacts in the scrubbing solution indirectly to form hydrogen carbonate and a protonated amine.
Undesirably, in an amine-containing scrubbing solution, in addition to carbon dioxide, other acid gases are also absorbed, in particular nitrogen oxides and/or sulfur oxides. In contrast to carbon dioxide, the further gases, however, with the amine-containing absorbents form, inter alia, heat stable salts (HSS) which can no longer be converted back in the desorption unit. These heat stable salts successively reduce the capacity of the scrubbing solution for absorption of carbon dioxide during operation owing to the decrease in amine concentration caused thereby. Furthermore, they may promote corrosion and adversely affect the flow properties of the scrubbing solution.